Using the Web Portal to enable Alexa on your ecobee thermostat:

You’ll be prompted to sign into your Amazon account to allow permission to link your Amazon and ecobee accounts

Click “Done”

Success! Your ecobee is now linked to Alexa! Your ecobee4 is now connected to Alexa with the "ecobee" skill and "ecobee plus" skills enabled.

NOTE: You'll need to "Discover Devices" for Alexa to recognize your ecobee4. To discover devices, say "Alexa, discover devices" to your ecobee4's speaker. If you are not near your thermostat at the time, you can always complete this step later.